Combustion appliances are known comprising at least one pump for feeding liquid fuel to at least one liquid-fuel atomizing head.
More specifically, burners or heating units have at least one pump for drawing fuel from a tank and feeding it under pressure to at least one nozzle where the liquid is atomized and made suitable for combustion (mechanical atomization).
Given the high pressures (5-20 bars) and low flow rates involved, the pumps are normally volumetric types.
Gear-type volumetric pumps are the most commonly used.
In pumps of this sort, one gear is driven by an electric motor, and in turn drives an idle second gear; and the liquid flows through an intake port into one of the cavities formed by the teeth of the two gears, and is discharged to the delivery port while gradually increasing in pressure.
The two gears and the seat in which they operate must be machined accurately to ensure optimum performance of the pump.
Moreover, the mechanical assembly defined by the two gears is lubricated by the pumped fuel itself.
When the pump is idle, however, fuel remains trapped inside the cavities formed by the geometry of the gears.
In certain conditions, and if the pump remains idle for any length of time, the fluid film deposited inside the cavities undergoes oxidation and/or polymerization, thus resulting in alteration of its properties, in particular viscosity.
When this happens, the fluid deposit may assume such a consistency as to prevent rotation of the gears when the pump is started up again.
That is, the starting torque of the electric motor is no longer enough to overcome the breakaway friction torque produced by the high-viscosity fluid deposit.
One of the causes of the problem is the addition of vegetable fuel to the mineral fuel.
Vegetable fuel may comprise unsaturated polymer chains, which, in the presence of oxygen or during prolonged stoppages, may combine to form macromolecules, the relative flow properties of which are greatly inferior to those of non-oxidized fuel.